[RFC][PATCH] locking/refcount: Provide __refcount API to obtain the old value
From: peterz
Date: Wed Jul 29 2020 - 07:11:48 EST
On Tue, Jul 28, 2020 at 11:56:38AM +0100, David Howells wrote:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> > > Please do not _undo_ the changes; just add the API you need.
> >
> > add_return and sub_return are horrible interface for refcount, which is
> > the problem.
> >
> > If you meant: refcount_dec(), but want the old value for tracing, you
> > want a different ordering than if you wanted to do
> > refcount_dec_and_test(); dec_return can't know this.
> >
> > David, would something like a __refcount_*() API work where there is a
> > 3rd argument (int *), which, if !NULL, will be assigned the old value?
>
> That would be fine, though the number needs to be something I can interpret
> easily when looking through the traces. It would also be okay if there was an
> interpretation function that I could use in the trace point when setting the
> variable.
I'm not entirely sure what you mean with interpret, provided you don't
trigger a refcount fail, the number will be just what you expect and
would get from refcount_read(). If you do trigger a fail, you'll get a
negative value.
How's the below? I didn't provide __refcount versions for the external
functions, I suppose that can be done too, but wondered if you really
needed those.
---
Subject: locking/refcount: Provide __refcount API to obtain the old value
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Wed Jul 29 13:00:57 CEST 2020
David requested means to obtain the old/previous value from the
refcount API for tracing purposes.
Duplicate (most of) the API as __refcount*() with an additional
'int *' argument into which, if !NULL, the old value will be stored.
Requested-by: David Howells <dhowells@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
include/linux/refcount.h | 65 +++++++++++++++++++++++++++++++++++++++++------
1 file changed, 57 insertions(+), 8 deletions(-)
--- a/include/linux/refcount.h
+++ b/include/linux/refcount.h
@@ -165,7 +165,7 @@ static inline unsigned int refcount_read
*
* Return: false if the passed refcount is 0, true otherwise
*/
-static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
+static inline __must_check bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
{
int old = refcount_read(r);
@@ -174,12 +174,20 @@ static inline __must_check bool refcount
break;
} while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
+ if (oldp)
+ *oldp = old;
+
if (unlikely(old < 0 || old + i < 0))
refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF);
return old;
}
+static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
+{
+ return __refcount_add_not_zero(i, r, NULL);
+}
+
/**
* refcount_add - add a value to a refcount
* @i: the value to add to the refcount
@@ -196,16 +204,24 @@ static inline __must_check bool refcount
* cases, refcount_inc(), or one of its variants, should instead be used to
* increment a reference count.
*/
-static inline void refcount_add(int i, refcount_t *r)
+static inline void __refcount_add(int i, refcount_t *r, int *oldp)
{
int old = atomic_fetch_add_relaxed(i, &r->refs);
+ if (oldp)
+ *oldp = old;
+
if (unlikely(!old))
refcount_warn_saturate(r, REFCOUNT_ADD_UAF);
else if (unlikely(old < 0 || old + i < 0))
refcount_warn_saturate(r, REFCOUNT_ADD_OVF);
}
+static inline void refcount_add(int i, refcount_t *r)
+{
+ __refcount_add(i, r, NULL);
+}
+
/**
* refcount_inc_not_zero - increment a refcount unless it is 0
* @r: the refcount to increment
@@ -219,9 +235,14 @@ static inline void refcount_add(int i, r
*
* Return: true if the increment was successful, false otherwise
*/
+static inline __must_check bool __refcount_inc_not_zero(refcount_t *r, int *oldp)
+{
+ return __refcount_add_not_zero(1, r, oldp);
+}
+
static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
{
- return refcount_add_not_zero(1, r);
+ return __refcount_inc_not_zero(r, NULL);
}
/**
@@ -236,9 +257,14 @@ static inline __must_check bool refcount
* Will WARN if the refcount is 0, as this represents a possible use-after-free
* condition.
*/
+static inline void __refcount_inc(refcount_t *r, int *oldp)
+{
+ __refcount_add(1, r, oldp);
+}
+
static inline void refcount_inc(refcount_t *r)
{
- refcount_add(1, r);
+ __refcount_inc(r, NULL);
}
/**
@@ -261,10 +287,13 @@ static inline void refcount_inc(refcount
*
* Return: true if the resulting refcount is 0, false otherwise
*/
-static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
+static inline __must_check bool __refcount_sub_and_test(int i, refcount_t *r, int *oldp)
{
int old = atomic_fetch_sub_release(i, &r->refs);
+ if (oldp)
+ *oldp = old;
+
if (old == i) {
smp_acquire__after_ctrl_dep();
return true;
@@ -276,6 +305,11 @@ static inline __must_check bool refcount
return false;
}
+static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
+{
+ return __refcount_sub_and_test(i, r, NULL);
+}
+
/**
* refcount_dec_and_test - decrement a refcount and test if it is 0
* @r: the refcount
@@ -289,9 +323,14 @@ static inline __must_check bool refcount
*
* Return: true if the resulting refcount is 0, false otherwise
*/
+static inline __must_check bool __refcount_dec_and_test(refcount_t *r, int *oldp)
+{
+ return __refcount_sub_and_test(1, r, oldp);
+}
+
static inline __must_check bool refcount_dec_and_test(refcount_t *r)
{
- return refcount_sub_and_test(1, r);
+ return __refcount_dec_and_test(r, NULL);
}
/**
@@ -304,12 +343,22 @@ static inline __must_check bool refcount
* Provides release memory ordering, such that prior loads and stores are done
* before.
*/
-static inline void refcount_dec(refcount_t *r)
+static inline void __refcount_dec(refcount_t *r, int *oldp)
{
- if (unlikely(atomic_fetch_sub_release(1, &r->refs) <= 1))
+ int old = atomic_fetch_sub_release(1, &r->refs);
+
+ if (oldp)
+ *oldp = old;
+
+ if (unlikely(old <= 1))
refcount_warn_saturate(r, REFCOUNT_DEC_LEAK);
}
+static inline void refcount_dec(refcount_t *r)
+{
+ __refcount_dec(r, NULL);
+}
+
extern __must_check bool refcount_dec_if_one(refcount_t *r);
extern __must_check bool refcount_dec_not_one(refcount_t *r);
extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock);